Solar Heat Gain Coefficient
From Swikipedia
Introduction
Solar Heat Gain Coefficient or SHGC is the fraction of external solar radiation that is admitted through a window or skylight, both directly transmitted, and absorbed and subsequently released inward.
Brief Description
Calculating SHGC:
The relationship between SHGC and SC (Shading coefficient) is as follows: SHGC = SC × 0.87. Shading coefficients measure the solar energy transmittance through windows. SHGC values range from 0 to 1, a lower value representing less solar heat transmission and greater shading ability. SC values are calculated using the sum of the primary solar transmittance (T-value) and the secondary transmittance. Primary transmittance is the fraction of solar radiation that directly enters a building through a window compared to the total solar insolation, the amount of radiation that the window receives. The secondary transmittance is the fraction of inwardly flowing solar energy absorbed in the window (or shading device) again compared to the total solar insolation.
Reducing SHGC:
SHGC can be reduced by applying a spectrally selective low-E (low-emissivity) coating to glazing. This type of low-E coating can reduce heat loss in the winter as well as solar gain in the summer. Reflective coatings and tinted glass can also help reduce the SHGC.
In passive solar design, south-facing windows with high SHGC ratings might be needed to provide a building with heat in the winter. But a properly designed roof overhang is typically used to reduce the solar heat gain from these windows in the summer.
References:
http://en.wikipedia.org/wiki/Solar_gain
http://www.daviddarling.info/encyclopedia/S/AE_solar_heat_gain_coefficient.html
